1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image by using an electronic printing technology, and particularly to an image forming apparatus which has a plurality of developing units for storing developers of the same color, and which performs calibration by means of a pattern of patches without reducing throughput in printing.
2. Description of the Related Art
An image forming apparatus which forms an image by using an electronic printing technology is provided in a printer, facsimile, copier and the like, and comprises an image supporting body (photoconductor drum) in which an electrostatic latent image is formed in accordance with image data, a charging unit for charging an external surface of the image supporting body, an exposure unit for exposing the external surface of the charged image supporting body in accordance with the image data to form the electrostatic latent image, a developing device for feeding a toner as a developer to the image supporting body to develop the electrostatic latent image of the image supporting body into a toner image, and a transfer unit for transferring the toner image to a medium as a target for transfer.
The developing device detachably holds the developing units containing a plurality of color toners or toners of the same color, and brings an appropriate developer proximate to the image supporting body in accordance with the developing timing. The developing device therefore has a developing rotary which is subjected to rotation control. When performing color printing, a plurality of color toners, e.g. four colors of developing units (yellow Y, magenta M, cyan C, and black K), are mounted on the developing rotary, and these developing units are sequentially brought proximate to the image supporting body to develop each color.
On the other hand, there has been suggested to mount the plurality of developing units of the same color all together, e.g. black, on the developing rotary of the developing device to obtain an image forming apparatus for black-and-white printing. Examples are shown in Japanese Patent Application Laid-Open Nos. 2002-351190 (published on Dec. 4, 2002) and 2003-316106 (published on Nov. 6, 2003). In this image forming apparatus for black-and-white printing, a plurality of black developing units can be mounted, thus, even when performing a large amount of black-and-white printing, frequency of replacing the developing units can be reduced by sequentially using the plurality of developing units.
In an image forming apparatus where an electrophotographic technology is used, an image supporting body (photoconductor drum) is charged to a predetermined bias potential, an exposure beam of a predetermined strength is emitted to the image supporting body to form a latent image, and a toner is transferred from a developing unit by means of the bias potential difference between the developing unit and the image supporting body. Therefore, how the toner is transferred differs according to control parameters such as the bias potential between the developing unit and image supporting body, the exposure strength and the like. Further, even with the same control parameter, how the toner is transferred differs along with a change in the external environment, an increase in the number of usages of the developing unit, replacement of the developing unit, and the like. How the toner is transferred has an affect on the concentration of a development pattern. Therefore, in general, calibration is performed appropriately in which a predetermined pattern of patches is formed on the image supporting body by means of a toner, and an optimum control parameter is determined in accordance with the optical concentration of the pattern of patches.